1. Field of the Invention
The present invention relates to a two-wire type converter in which voltage signals detected by a detecting element of a pH meter, for example, are converted into current signals and the resulting current signals are transferred to an indicator of a measuring device or the like through a two-wire type transmission channel which also serves as a feed line of electric power for driving said converter. In particular, the present invention relates to a two-wire type insulated converter having a construction in which said converter is electrically separated and insulated into a system physically located near said detecting element and a system physically located far from said detecting element, said converter separarated by means of an insulated transformer so as to prevent measurement errors due to ground loop currents.
2. Description of the Prior Art
The separation of said converter into said circuit system physically located near said detecting element and said circuit system physically located far from said detecting element requires the modulation of voltage signals in order to transfer the information corresponding to said voltage signals of said detecting element from said circuit system near said detecting element to said circuit system far from said detecting element. Prior art systems include an amplitude modulation type system, as disclosed by Japanese Patent Publication No. 44,479/1979 (hereinafter referred to as an AM modulation type system) and a frequency modulation type system, as disclosed by Japanese Laid-Open Patent No. 137,368/1979 (hereinafter referred to as an FM modulation type system). However, in an AM modulation type system, said voltage signals output from said detecting element are transmitted in the form of changes in the carrier signal amplitude. It is, therefore, necessary that the unmodulated amplitude of the carrier signal be maintained at a constant level and not disturbed by changes in the system power source, or changes in system temperatures or the like. To this end, a sufficiently large electric power must be always supplied. This leads to a defect in that the power consumption is large. On the contrary, in an FM modulation type system, said voltage signals output from said detecting element are transmitted in the form of changes in the carrier signal frequency. It is therefore not necessary to keep the amplitude of the carrier stable as in an AM modulation type system. However, an FM modulation type system also has a defect in that the transmission efficiency of an insulated transformer and the power consumption of such a circuit are changed owing to the changes of the carrier frequency and the linearity characteristics are thereby deteriorated. In addition, such a system inevitably requires the use of special and expensive parts such as a voltage-frequency conversion circuit.